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DO NOT TRADE
Scan for malware https://www.malwarebytes.com/
Deauthorize all other devices https://store.steampowered.com/twofactor/manage
Change passwords from a clean computer
Generate new backup codes for your Mobile App
Revoke the API key https://steamcommunity.com/dev/apikey (there should be nothing in the APIKEY)
Please review how you are logging into Steam, you somehow gave them your log in information. This could of been due to the computer being compromised and redirecting to a fake login, or you using a 3rd party site to login to steam.
Once you are sure you have regained control, you can start trading again. Please triple check any trades you make to be sure they are going to the correct account.
if you dont have access to the account anymore, then you need to do an account recovery
most of the account are throw away accounts - either stolen or created just to launder items.